Microelectromechanical systems (MEMS) have been used in a wide range of applications; from pressure sensors and accelerometers to microphones and digital displays. In 2006, STMicroelectronics and Nintendo revolutionized the entire MEMS industry through the launch of the Nintendo Wii gaming console that uses 3-D MEMS accelerometers for motion control. Since then, MEMS devices have been used in almost all new technology from smart phones to tablet PCs. In 2010, the MEMS industry experienced a 25% growth with the top four MEMS suppliers, Texas Instruments, Hewlett-Packard, Robert Bosch, and STMicroelectronics, increasing MEMS sales by 37%.
A wide variety of the physical properties of materials, such as ferroelectricity, ferromagnetism, piezoelectricity, conductivity, and dielectric permittivity depend upon material anisotropy and are therefore strongly affected by crystallographic texture, as reported in M. D. Vaudin, et al., “Accuracy and Reproducibility of X-ray Texture Measurements on Thin Films,” Mat. Res. Soc. Symp. Proc., Vol. 721, entitled “Magnetic and Electronic Films-Microstructure, Texture and Application to Data Storage,” edited by P. W. DeHaven et al. (Mat. Res. Soc., Warrendale Pa., 2002) pp. 17-22. With the appropriate choice of thin film texture, device operating efficiency and reliability can be strongly affected. Therefore texture is one critical factor for thin film process control and is fundamental to device reproducibility
Lead zirconate titanate (Pb(ZrxTi1-x)O3 or PZT) exhibits piezoelectric properties in thin films and is the most widely used piezoelectric bulk ceramic with ferroelectric properties. Examples of the use of thin films of PZT (used to create large force, large displacement actuators) include actuators for RF switches, relays and inkjet print heads.
To date, two approaches have been taken to produce lead zirconate titanate (PZT) thin film devices with the spontaneous polarization normal to plane of the film and thus normal to the planar capacitor device, i.e. {001}-orientation. It is noted that directions in crystal lattices are defined in terms of directions l, m, and n, known as the Miller indices. Indices {1,0,0}, {0,1,0} and {001} represent planes orthogonal (normal) to the l, m, and n directions, respectively. The crystallographic directions are lines linking nodes (atoms, ions or molecules) of a crystal. Similarly, the crystallographic planes are planes linking nodes. Some directions and planes have a higher density of nodes; these dense planes have an influence on the behavior of the crystal. The notation {001} denotes the set of all planes that are equivalent to (001) (as shown in FIG. 2) by the symmetry of the lattice. Heteroepitaxial growth makes use of a crystal substrate to initiate growth of an overlying crystalline material that has a different crystal structure than the substrate. Either a polar or a non-polar substrate may be used to initiate growth of a polar film. Further discussion of spontaneous polarization is found in FIG. 8 of U.S. Patent Application No. 2010/0006780 and U.S. Pat. No. 7,956,369.
The piezoelectric coefficient of PZT is inherently linked to its crystalline quality. The highest magnitude piezoelectric coefficients are observed at the PZT morphotropic phase boundary (MPB) (See FIG. 1), where the crystal structure changes abruptly between the tetragonal and rhombohedral symmetry. The MPB is located at PbZr0.52Ti0.48O3, or PZT (52/48), and is the composition in which both the dielectric permittivity and piezoelectric coefficients reach a maximum. In thin film form, the composition and the crystalline texture must be controlled to achieve the maximum piezoelectric coefficients. For PZT (52/48), the highest coefficients are reported for a {001} textured PZT (52/48). The increased piezoelectric response and poling efficiency near to x=0.52 is due to the increased number of allowable domain states at the MPB. At this boundary, the 6 possible domain states from the tetragonal phase <100> and the 8 possible domain states from the rhombohedral phase <111> are equally favorable energetically, thereby allowing a maximum 14 possible domain states.
FIG. 3 shows the orientation of a polar material relative to the growth direction. One approach to producing PZT thin film devices with the spontaneous polarization normal to the plane of the film is to use a single crystal substrate and grow epitaxial layers of the bottom electrode and ferroelectric layer. The difficulty with this approach is that it places very strict requirements on the single crystal substrate, and the electrode must provide an epitaxial relationship with both the single crystal substrate and the ferroelectric film. The second approach to obtaining {001}-orientation is to use a seed layer and/or variations in process conditions to produce a {001}-textured PZT film whereby the PZT {001}-planes lie parallel to the substrate plane, but the relative orientation of the grains are randomly rotated about the substrate normal and there is no defined crystallographic relationship between the capacitor electrodes and the PZT.
Previous publications and patents do not appear to provide a complete description of the {001}-texture inasmuch as it is insufficient to just state independently just the percentage of {100}-textured grains or {001}-textured grains or angular distribution width of the textured grains. In order to fully define a texture and the crystallographic efficiency (or figure of merit) of the texture, the volume fraction of the {100}-oriented grains relative to the total volume of the PZT film should be defined. Of that volume fraction, then the volume fraction of the {001}-oriented grains must be defined. And finally, the misalignment distribution of the {001}-oriented grains relative to the substrate normal (which is also the electric field direction of the device) must be defined. When these crystallographic parameters are defined, a figure-of-merit for the material can be calculated.
U.S. Pat. No. 6,682,772, to Fox, et al, entitled “High temperature deposition of Pt/TiOx for bottom electrodes,” hereby incorporated by reference, has been reported for a bottom electrode consisting of TiO2/{111}-textured Pt which resulted in improved PZT ferroelectric capacitor electrical characteristics for FRAM applications.